1. Field of the Invention
The invention relates to the technical field of on-screen displays (OSDs) and, more particularly, to an on-screen display device with compressed data and the display method thereof.
2. Description of Related Art
Televisions have become a popular household appliance for every family, and have developed with the advance electronic technologies to thus provide enhanced image qualities and more program channels. In addition, an on-screen display (OSD) device is applied to a TV screen in order to show certain specific fonts or graphics carrying with messages on the screen to a viewer. When the viewer operates the TV to change the channel, volume, quality, or the like, current states are accordingly displayed on the TV screen and shown to the viewer. Therefore, the OSD device has become an important message generator implemented in a human-machine interface.
A typical OSD configuration can be divided into bitmap base and font base. A bitmap-based OSD device can provide a user interface with more graphic images, which requires a larger memory capacity. A font-based OSD device provides a convenient software development and requires a smaller memory capacity, but it can only provide simple OSD frames.
FIG. 1 is a block diagram of a typical bitmap-based OSD device. As shown in FIG. 1, the ROM 110 stores the OSD data to be displayed. The processor 120 processes and combines the OSD data to be displayed into a bitmap data, and stores the bitmap data in the buffer 140 through the buffer controller. The processor 120 also processes the display information for the bitmap data. The OSD display engine 150 performs a color plate correspondence to the bitmap data stored in the buffer 140 in accordance with the display information, and accordingly produces an OSD image display data. The mixer 160 mixes the OSD image display data and a video image for displaying on a display.
When a 512×200 bitmap image containing 16 colors is displayed, the ROM 110 requires a memory capacity of 50 Kbytes ((512×200 pixels×4 bits/pixel)/8 bits) to store the bitmap image. For the users, the quality of current TV operating interfaces is desired higher and higher, so that the bitmap-based OSD device requires a larger external memory for storing the increasingly complex OSD images, which increases the cost.
To overcome this problem, the U.S. Pat. No. 5,969,770 granted to Horton entitled “Animated on-screen display provisions for an MPEG video signal processing system” uses an MPEG approach to compress an animated OSD image stream and store the compressed image stream in the ROM. When the compressed image stream is to be displayed, a multiplexer sends the compressed image stream to an MPEG decoder in a predetermined period for decompression and output through a video channel. Such a way can save the memory space but cannot apply to a TV or a display without the MPEG decompressor.